An example of such power supply apparatuses is disclosed in JP 05-111244 A. The power supply apparatus disclosed in this Japanese publication includes a rectifier which rectifies a commercial AC power supply voltage, and the resultant rectified voltage is applied to a smoothing capacitor through a reactor. A voltage-boosting switching device is connected in the input of the smoothing capacitor. The charging time of the smoothing capacitor is controlled by controlling the ON and OFF times of the switching device. Thus, even when a commercial supply voltage of a different magnitude is applied to the apparatus, the input voltage to the smoothing capacitor can be made substantially constant through the appropriate control of the ON and OFF times of the voltage-boosting switching device. Accordingly, this power supply apparatus can be used with commercial power supply voltages of different magnitudes.
The smoothed voltage from the smoothing capacitor is applied to a high-frequency switching circuit, where it is converted to a high-frequency AC voltage, and the resultant high-frequency voltage is voltage-transformed by an output transformer. The voltage-transformed high-frequency voltage is, then, converted to a DC voltage, and the resultant DC voltage is applied, as it is or after it is re-converted to an AC voltage, to an arc load.
The above-described type of power supply apparatus is used in various countries and areas. In some places, commercial AC power supply voltages may be unstable because of, for example, unstable power supply ability. If the commercial AC power supply voltage applied to the power supply apparatus increases much, an excessive voltage is applied to the output transformer, causing its core to be saturated. Then, appropriate power can no longer be supplied to the load. In order to avoid such problem, a transformer with a magnetic saturation characteristic having some margin has been employed. However, the use of such output transformer having an unnecessary margin increases the size and weight of the power supply apparatus with such transformer. In addition, such power supply apparatus becomes expensive.
An object of this invention is to provide a power supply apparatus including an output transformer having suppressed magnetic saturation, which is smaller in size, lighter in weight and lower in cost than prior art apparatuses.